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Old May 6th 19, 12:15 PM posted to sci.space.policy
Jeff Findley[_6_]
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Default SpaceX Dragon 2 capsule destroyed in abort motor ground test

In article ,
says...

On 2019-05-05 09:43, Jeff Findley wrote:

Depends on the details of the design. Plumbing, control valves, check
valves, and etc. will all cause a pressure drop from the tanks to the
combustion chamber.


OK, let me reformulate the question:

Would there be an expectation that the tanks holding hypergolics be
built to widthstand operating pressure (say 1000psi) with some safety
margin, or would they be designed to support the same pressure as what
the helium tanks are built for (say 10,000psi) in case a regulator fails
and equalizes between helium of hypergolic tanks ?


I believe Fred already answered this.

This another reason why I hate idle speculation. You're essentially
trying to reverse engineer the entire design to come up with pressures
of everything.


No, trying to understand how these engines are built so that I can see
various way they can fail and better understand the meaning of what
SpaceX does say.

For instance, by being told here that the Draco and SuperDracos are
separate systems operating are different pressures and from different
tanks, it put into perspective the statements from the SpaceX engineer
about why they weren't worried about Dragon 1's launch even if they have
similar Draco engines.


Because the systems are completely separate and the test of the Dracos
passed. It was the test of the Super Dracos that failed. Therefore,
using basic logic and reasoning, the Dracos on both Dragon and Dragon 2
are just fine. The problem is with the Super Dracos on Dragon 2.

BTW, Dragon 2 was just grabbed by the SSRMS. I was watching the NASA TV
live-stream on YouTube. Once they get Dragon 2 into the hold position,
the SSRMS moves relatively fast to grab the thing. Amazing really.

"boom". This entire system is supposed to be fast, reliable, and fail
safe.


Sometimes one has to make compromises to maintain the balance of fast,
reliable and fal safe. And sometimes you forget a possible failure mode
or are told to not worry about it because component X has never failed
and considered reliable.


Compromises? No, it's got to be fast, reliable, and fail safe. Those
simple terms are better captured by the actual engineering requirements
specifications for the Super Draco *system*. It either meets the specs
or it doesn't. So when something fails during testing, you figure out
why, fix it, and move forward.

Depends on the design. There could be something like a blow off valve
to vent helium in case of over-pressurization of the plumbing.


Would it be safe to have a blow off valve between helium tank and
hypergolic tank or would this have too high odds of hypergolic liquid
venting? (or is that considered a necessary evil to prevent hypergolic
tank from rupturing?


I don't even know if the system has a blow off valve. Because if that
valve did activate, you've got bigger problems (i.e. your escape system
isn't going to pull you away from the fireball engulfing a failing
Falcon launch vehicle).

In terms of pressurizing the helium tank, wouldn't there be a blow off
on pad equipment instead of being on-board since overfilling can only
happen on pad? (ag: not have to carry weight of blow off valve for
whole flight when it is only needed while on pad when tanks are filled.


From what I could see from the cell phone video, there wasn't any "pad
equipment" attached to Dragon 2 during this test. It was just sitting
on top of a test fixture.

Jeff
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